MCAT Biochemistry Review

Chapter 8: Biological Membranes

8.4 Specialized Membranes

The membranes of most organelles are similar to the cell membrane in both composition and general characteristics; however, it is important to note that some membranes are specialized to accomplish specific functions. For instance, the sarcolemma of muscle cells must maintain a membrane potential for muscle contraction to occur. Membrane composition may also be altered slightly, especially in the case of mitochondria.

MEMBRANE POTENTIAL

The impermeability of the cell membrane to ions and the selectivity of ion channels both lead to an electrochemical gradient between the exterior and interior of cells. The difference in electrical potential across cell membranes is called the membrane potential, Vm. The resting potential for most cells is between –40 and –80 mV, although the potential can rise as high as +35 mV during depolarization of the cell. Maintaining membrane potential requires energy because ions may passively diffuse through the cell membrane over time using leak channels; therefore, an ion transporter or pump such as the sodium–potassium pump (Na+/ K+ATPase) regulates the concentration of intracellular and extracellular sodium and potassium ions. Chloride ions also participate in establishing membrane potential. The Nernst equation can be used to determine the membrane potential from the intra- and extracellular concentrations of the various ions:

Equation 8.2

where R is the ideal gas constant, T is the temperature in kelvins, z is the charge of the ion, and F is the Faraday constant ( ). The simplification to 61.5 in the numerator assumes body temperature, 310 K. The Goldman–Hodgkin–Katz voltage equation flows from the equation, taking into account the relative contribution of each major ion to the membrane potential:

Equation 8.3

where P represents the permeability for the relevant ion. Note that chloride is inverted relative to the other ions because it carries a negative charge.

BRIDGE

The cell membrane is often compared to a capacitor because opposite charges are maintained on either side of the membrane. Capacitance is discussed in Chapter 6 of MCAT Physics and Math Review.

Sodium–Potassium Pump

There is a steady-state resting relationship between ion diffusion and the Na+/K+ATPase. One of the main functions of the Na+/K+ ATPase is to maintain a low concentration of sodium ions and high concentration of potassium ions intracellularly by pumping three sodium ions out for every two potassium ions pumped in. This movement of ions removes one positive charge from the intracellular space of the cell, which maintains the negative resting potential of the cell. As mentioned before, the cell membrane also contains leak channels that allow ions, such as Na+ and K+, to passively diffuse into or out of the cell down their concentration gradients. Cell membranes are more permeable to K+ ions than Na+ ions at rest because there are more K+ leak channels than Na+ leak channels. The combination of Na+/K+ ATPase activity and leak channels together maintain a stable resting membrane potential.

MITOCHONDRIAL MEMBRANES

Mitochondria are referred to as the “powerhouse” of the cell because of their ability to produce ATP by oxidative respiration. Mitochondria contain two membranes: the inner and outer mitochondrial membranes.

Outer Mitochondrial Membrane

The outer mitochondrial membrane is highly permeable due to many large pores that allow for the passage of ions and small proteins. The outer membrane completely surrounds the inner mitochondrial membrane, with the presence of a small intermembrane space in between the two layers.

Inner Mitochondrial Membrane

The inner mitochondrial membrane has a much more restricted permeability compared to the outer mitochondrial membrane. Structurally, the inner mitochondrial membrane contains numerous infoldings, known as cristae, which increase the available surface area for the integral proteins associated with the membrane. These proteins, discussed in Chapter 10 of MCAT Biochemistry Review, are involved in the electron transport chain and ATP synthesis. The inner membrane also encloses the mitochondrial matrix, where the citric acid cycle produces high-energy electron carriers used in the electron transport chain. The inner mitochondrial membrane contains a very high level of cardiolipin and does not contain cholesterol.

MCAT Concept Check 8.4:

Before you move on, assess your understanding of the material with these questions.

1. How is the resting membrane potential maintained?

2. Given the following data, calculate the resting membrane potential of this cell:

Ion

Permeability (Relative)

Intracellular Concentration

Extracellular Concentration

Na+

0.05

14 mM

140 mM

K+

1

120 mM

4 mM

Cl−

0

12 mM

120 mM

3. What distinguishes the inner mitochondrial membrane from other biological membranes? What is the pH gradient between the cytoplasm and the intermembrane space?